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vb Instrument Measurement Time

 

QUESTION

How long does the vb instrument take to measure?

ANSWER

The key to calculating how long a particular parameter set will take to measure is the frequency resolution that you are achieving. If the line spacing is 1 Hz, then the instrument will be sampling for 1 second. The instrument calculates FFTs almost instantaneously (about 100 microseconds). The time that it takes to take a measurement with the instrument depends on the time required to capture the vibration signal in the way that you want to measure it.

To illustrate, a 3200 line FFT takes 100 microseconds to calculate; however, the data may take anywhere up to 32 seconds to collect (Fmax = 100 Hz), dependent on the measurement parameters.

As an example, a 400 line spectrum with a maximum frequency of 400 Hz has a line spacing of 1 Hz, and will take 1 second. 1600 lines and a maximum frequency of 1600 Hz will also take 1 second. Even though more data is collected and processed for the 1600 Hz 1600 line recording, the sample rate is higher, so it takes the same time to collect the data.

To calculate the full time for measurement, the number of averages and overlap must also be taken into account.

The full formula is:
Time = lines/fmax*((averages-1)*(1-overlap) + 1)